Heimdal libheimntlm library is a implementation of the NTLM protocol, both version 1 and 2. The GSS-API mech that uses this library adds support for transport encryption and integrity checking.
NTLM is a protocol for mutual authentication, its still used in many protocol where Kerberos is not support, one example is EAP/X802.1x mechanism LEAP from Microsoft and Cisco.
This is a support library for the core protocol, its used in Heimdal to implement and GSS-API mechanism. There is also support in the KDC to do remote digest authenticiation, this to allow services to authenticate users w/o direct access to the users ntlm hashes (same as Kerberos arcfour enctype keys).
More information about the NTLM protocol can found here http://davenport.sourceforge.net/ntlm.html .
The Heimdal projects web page: http://www.h5l.org/
Example to to use test_ntlm::c .